Add real desktop icons plus right-click desktop/icon context menus

Closes "right-click on bare desktop" - previously a true no-op, nothing
rendered above the wallpaper at all. Requested directly: a real desktop
"just like windows does" - Home/Computer/Trash plus one icon per real
~/Desktop entry, individually draggable with persisted grid positions,
double-click to open, right-click menus (per-icon "Open" plus "Set as
Wallpaper" for image files when general.wallpaper_command is set; bare
desktop "New Folder"/"Refresh").

Architecture mirrors the existing context_menu.rs/snap_flyout.rs
"compositor-owned floating UI" pattern: desktop_icons.rs (data model, grid
layout, filesystem scan, hit-test), desktop_icons_state.rs (JSON
persistence, same shape as monitor_layout.rs), desktop_menu.rs (the new
right-click menu, reusing decoration::render_context_menu's existing
rasterizer), state/desktop_icons.rs (CompState glue: rescan/select/drag/
open/persist), and a new decoration::render_desktop_icon rasterizer --
hand-drawn glyphs, since no PNG/SVG decoding capability exists anywhere in
this workspace. Wired into both render loops (udev and winit) above the
wallpaper and below every window, and into input/pointer.rs's button/
motion handlers for selection, drag, double-click, and both menus.

Four new config keys: general.desktop_icons (default true - a directly
requested, purely visual feature, unlike the opt-in-while-experimental
general.gpu), general.file_manager, general.desktop_icon_single_click,
general.wallpaper_command (all default off/empty).

Deliberately out of scope for this pass, stated up front: move-to-trash
and "Empty Trash" (destructive, no confirmation-dialog primitive to gate
them on yet), filesystem watching, multi-select, per-mimetype icon art,
icons on any monitor but the primary one.

124 wayland-crate tests (up from 106), full workspace build and clippy
clean.

1 files changed, 105 insertions, 0 deletions

diff --git a/crates/wayland/src/desktop_icons_state.rs b/crates/wayland/src/desktop_icons_state.rs
new file mode 100644
index 0000000..964c316
--- /dev/null
+++ b/crates/wayland/src/desktop_icons_state.rs
@@ -0,0 +1,105 @@
+//! Persists desktop icons' grid cells across restarts - same shape as
+//! `monitor_layout.rs`, deliberately not sharing code with it (its own doc
+//! comment gives the reasoning: two small, independent files beat a
+//! cross-module dependency for four lines of env-var lookup).
+//!
+//! Only icons the user has actually dragged get an entry here - see
+//! `desktop_icons::rescan`'s own doc comment for why a fresh/unmoved icon
+//! deliberately has no row in this file at all.
+
+use std::collections::HashMap;
+use std::path::PathBuf;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, Default)]
+struct PersistedIcons {
+    /// Keyed by `DesktopIcon::id` - `"home"`/`"computer"`/`"trash"`, or a
+    /// real `~/Desktop` filename.
+    icons: HashMap<String, (i32, i32)>,
+}
+
+fn state_dir() -> PathBuf {
+    if let Ok(p) = std::env::var("SRDWM_STATE_PATH") {
+        return PathBuf::from(p);
+    }
+    if let Ok(xdg) = std::env::var("XDG_STATE_HOME") {
+        return PathBuf::from(xdg).join("srd");
+    }
+    if let Ok(home) = std::env::var("HOME") {
+        return PathBuf::from(home).join(".local/state/srd");
+    }
+    PathBuf::from("state/srd")
+}
+
+fn icons_path() -> PathBuf {
+    state_dir().join("desktop-icons.json")
+}
+
+/// Every remembered icon cell, by icon id. Empty (not an error) if the file
+/// doesn't exist yet or is unreadable/corrupt - a bad state file degrades
+/// to "every icon gets its default grid slot", not a startup failure.
+pub(crate) fn load() -> HashMap<String, (i32, i32)> {
+    let path = icons_path();
+    let Ok(bytes) = std::fs::read(&path) else { return HashMap::new() };
+    match serde_json::from_slice::<PersistedIcons>(&bytes) {
+        Ok(persisted) => persisted.icons,
+        Err(e) => {
+            log::warn!("desktop_icons_state: couldn't parse {path:?} ({e}); starting with default icon positions instead");
+            HashMap::new()
+        }
+    }
+}
+
+/// Overwrites one icon's remembered cell and rewrites the whole file --
+/// read-modify-write, same reasoning as `monitor_layout.rs::save_output`:
+/// a drag-and-drop is a rare, human-paced event, not a per-frame one, so
+/// re-reading the small file each time costs nothing.
+pub(crate) fn save_icon(id: &str, cell: (i32, i32)) {
+    let mut persisted = PersistedIcons { icons: load() };
+    persisted.icons.insert(id.to_string(), cell);
+    let dir = state_dir();
+    if let Err(e) = std::fs::create_dir_all(&dir) {
+        log::warn!("desktop_icons_state: couldn't create {dir:?} ({e}); this icon's position won't survive a restart");
+        return;
+    }
+    let Ok(bytes) = serde_json::to_vec_pretty(&persisted) else { return };
+    let path = icons_path();
+    // `.tmp`-sibling-then-rename, same crash-safety reasoning as
+    // `monitor_layout.rs::save_output`.
+    let tmp = path.with_extension("json.tmp");
+    if let Err(e) = std::fs::write(&tmp, &bytes) {
+        log::warn!("desktop_icons_state: couldn't write {tmp:?} ({e}); this icon's position won't survive a restart");
+        return;
+    }
+    if let Err(e) = std::fs::rename(&tmp, &path) {
+        log::warn!("desktop_icons_state: couldn't rename {tmp:?} to {path:?} ({e}); this icon's position won't survive a restart");
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Same reasoning as `monitor_layout.rs`'s own tests: only the pure JSON
+    // round-trip is exercised here, not `load()`/`save_icon()` themselves,
+    // since both touch real environment variables and the filesystem that
+    // parallel `cargo test` runs can't safely share.
+    #[test]
+    fn a_persisted_icon_map_survives_a_json_round_trip() {
+        let mut icons = HashMap::new();
+        icons.insert("home".to_string(), (0, 0));
+        icons.insert("report.txt".to_string(), (2, 3));
+        let persisted = PersistedIcons { icons };
+        let bytes = serde_json::to_vec(&persisted).unwrap();
+        let parsed: PersistedIcons = serde_json::from_slice(&bytes).unwrap();
+        assert_eq!(parsed.icons.get("home"), Some(&(0, 0)));
+        assert_eq!(parsed.icons.get("report.txt"), Some(&(2, 3)));
+    }
+
+    #[test]
+    fn corrupt_json_falls_back_to_an_empty_map_not_an_error() {
+        let result = serde_json::from_slice::<PersistedIcons>(b"not valid json");
+        assert!(result.is_err(), "sanity: this fixture must actually fail to parse");
+    }
+}